Conventional once-through boilers for variable pressure operation, as illustrated in FIG. 1, use furnace wall tubes consisting of spiral generating tubes which are inclined at gentle angles in the lower part of the furnace including the burner sections where the heat absorption is the highest in the furnace. In such a spirally wound boiler, all the generating tubes extend uniformly through the furnace regions where the heat absorption is high and the regions where the absorption is low. Consequently, there is little variation in absorption of heat by those generating tubes. With a uniform length, the tubes maintain a constant flow rate throughout, and the fluid temperature at the exit of the furnace shows a quite uniform distribution over the entire surrounding walls of the furnace.
In a boiler of the simultaneous upward flow type whose water-cooled walls consist of vertical tubes, unlike the counterparts of the spirally-wound monotube boilers, the generating tubes are so arranged that the fluid in some of the tubes passes through only the furnace regions where the heat absorption is high and in the other tubes passes through only the regions of low heat absorption. Naturally, the generating tubes show irregularities in heat absorption. Especially in the boiler equipped with corner-firing burners, the heat absorption is high in the center and low at the corners of the furnace. In view of this, a modified design employs water-cooled walls each consisting of a bank of generating tubes welded in parallel to a panel form, and, in order to avoid the development of excess thermal stresses in the water-cooled walls, an orifice is formed at the inlet of each generating tube or at the inlet of each distributing tube for each group of several generating tubes, and the rate of fluid flow in each tube is regulated by means of the orifice. In this way, in the boiler fitted with the corner-firing burners, as indicated by a full line in FIG. 2, the fluid temperatures at the outlets of the generating tubes at the exit of the furnace are kept substantially uniform over the entire surrounding walls of the furnace. The chain-line curve in FIG. 2 represents the temperature distribution obtained in the same manner but with generating tubes free of orifices.
Thus, the corner-fired boiler for constant-pressure operation, capable of maintaining a constant pressure in the furnace regardless of variation in load, offers an outstanding advantage that, as can be seen from FIG. 3, the pattern of heat absorption does not change in the width direction of the furnace. The use of orifices permits the maintenance of substantially the same temperature at the outlets of water-cooled walls of the furnace despite changes in working load and operating conditions.
However, with a boiler of the type which gives dry steam at the outlets of the water-cooled walls over a broad range of pressure changes, from the supercritical pressure down to a low pressure of about 80 kg/cm.sup.2 G as in a pressure-enthalpy chart of FIG. 4, changes in load are accompanied by considerable changes in the ratio of the specific volume at the inlets of generating tubes of the furnace, V.sub.i, to the specific volume at the outlets, V.sub.o, as indicated in FIG. 5. Therefore, although the ratio in heat absorption rate of highly heat-absorptive central region of the furnace to the corner regions where less heat is absorbed does not change under all load conditions to be encountered, it is not always easy to maintain uniform fluid temperature distribution at the outlets of the water-cooled walls of the furnace throughout the entire load range by means of only one type of fixed orifices. Especially in the low-pressure region, even a relatively slight difference in heat absorption will put the highly heat absorptive generating tubes into the superheated steam region and the less absorptive generating tubes in the wet steam region. As a consequence, the temperature difference between the two is widened to a disadvantage in that the planer panel stresses that result from thermal stressing are too high to build a single water-wall tube panel.
As explained, with the vertical tube type boiler for variable pressure operation, which works with its furnace generating tube outlets in the dry steam region over the broad pressure range from the supercritical region under heavy load down to 80 kg/cm.sup.2 G under light load, it is no longer satisfactory to rely merely upon orifice means for the control of heat absorption.
For the solution of these problems, the permissible minimum pressure for the operation of the boiler must be limited; for example, it must be raised to a higher pressure level. However, this means a corresponding rise of the lower limit for the operating pressure under part load, which will lead to a greater rate of turbine heat consumption due to an increased power requirement for pumping feedwater to the boiler. The outcome is not desirable from the economy-saving standpoint.